The packing density in packages for LSIs has been increased by three-dimensional packaging, etc., and progress has been made in reducing the thickness of wafers to achieve a wafer thickness of about 25 μm at the time of completion of the process. Power devices such as insulated-gate bipolar transistors (IGBTs) and MOS-type field effect transistors (MOSFETs) are being widely used as semiconductor switches in inverter circuits for industrial motors, vehicle motors, etc., power supplies for large-capacity servers, and uninterruptible power supplies. Semiconductor substrates for such power semiconductor devices are machined to a reduced thickness for the purpose of improving energization performance characteristics typified by an on-characteristic. In recent years, semiconductor devices have been manufactured by using, for improvements in terms of cost and characteristics, an extremely-thin-wafer process in which the thickness of a wafer manufactured by a floating zone (FZ) method is reduced to about 50 μm.
Wet etching or dry etching for removing work strain caused by grinding in a way of back grinding or polishing or by mechanical polishing is ordinarily used for machining a wafer to a reduced thickness. Forming of a diffusion layer on the back surface side by means of ion implantation and a heat treatment is performed and forming of an electrode is thereafter performed, for example, by means of sputtering. Under such circumstances, the frequency of occurrence of wafer breakage in machining of the back surfaces of wafers has been increased. In recent years, therefore, machining methods of machining a wafer so that only a central portion of the wafer is thinned while leaving a peripheral portion thick as a rib have been proposed for making wafers small in thickness (see, for example, PTL 1).
If a wafer with such a rib is used, a markedly high effect of limiting a warp of the wafer can be achieved and the facility with which the wafer is transported in a processing apparatus can be improved. Further, in handling of the wafer, the wafer has a markedly improved strength and the occurrence of cracking or chipping of the wafer can be reduced. A method has also been proposed which provides, in a wafer with such a rib, a transition region where the wafer is gradually reduced in thickness from the rib to the thinned portion, whereby wafer breakage is prevented in a step of heat treatment on the wafer with the rib (see, for example, PTL 2).